In recent years, an automatic adjustment function of a control system in an optical writing/reading apparatus has been developed by introducing a microcomputer. Conventionally, there is an optical writing/reading apparatus disclosed in Japanese Published Patent Application No. Hei. 9-320067, wherein the power of a laser applied to a recording medium is alternately switched between the power at reading and the power at erasing, and a top peak value and a bottom peak value of an error signal which is outputted from an AGC (Auto Gain Control) at this power switching are detected by a peak hold circuit, and then offset adjustment is performed to remove a difference between these peak values.
Hereinafter, a description will be given of an optical disk drive which performs reading, writing, or erasing of an optical disk, for example, a CD-R (Compact Disc-Recordable) or a CD-RW (Compact Disc-ReWritable), as a conventional optical writing/reading apparatus.
FIG. 11 is a block diagram illustrating the construction of a conventional optical disk drive.
An optical pickup 1 is composed of optical elements such as a laser light source like a laser diode, a collimator lens, an objective lens, a polarization beam splitter, a cylindrical lens, and the like, and a photodetector (not shown) having light-receptive parts of predetermined patterns, and the objective lens is driven by a two-axis actuator (not shown).
The optical pickup 1 detects a reflected light of a laser which is applied to a target track on an optical disk 3 being rotated by a spindle motor 2. The detected signal includes a reading signal which is obtained during reading on the basis of the amount of the reflected laser light from the optical disk 3, and address information by wobbling grooves.
An error signal generation unit 4 generates a focus error (FE) signal by the astigmatism method on the basis of the signal supplied from the optical pickup 1, generates a tracking error (TE) signal by the push-pull method, and outputs these servo error signals SE1 to an offset adjustment unit 5.
The offset adjustment unit 5 adds or subtracts an offset adjustment value stored in a storage means 7 to/from the servo error signal SE1 supplied from the error signal generation unit 4 to cancel an offset of the servo error signal SE1, and outputs the offset-canceled servo error signal SE2.
An AGC 15 normalizes the servo error signal SE2 received from the offset adjustment unit 5, on the basis of a signal indicating the sum total of the amount of light detected by the photodetector, and outputs the normalized servo error signal SE3 to a control unit 9.
The control unit 9 determines the characteristics of a servo loop, such as phase compensation of the servo error signal SE3 and the like, and outputs the servo error signal SE4 to a driver 10.
The driver 10 is a circuit for driving focusing and tracking actuators. On receipt of the servo error signal SE4 from the control unit 9, the driver 10 performs focusing and tracking control so as to bring the objective lens in the optical pickup 1 into an optimum state with respect to the optical disk 3.
A laser power switching circuit 12 changes the laser power emitted from the optical pickup 1, thereby performing reading/erasing of data from the optical disk 3, or writing of data into the optical disk 3.
An offset detection unit 6 has a peak hold circuit. The offset detection unit 6 detects the servo error signal SE1 when the laser power switching circuit 12 switches the laser power from the optical pickup 1 alternately between the reading state and the erasing state. The peak hold circuit (not shown) detects a top peak value and a bottom peak value of the servo error signal SE1 in the reading or writing state of the optical disk 1, and stores these values in the storage means 7. Further, the offset detection unit 6 detects a difference in the servo error signals between the reading state and the writing state of the optical disk 3 on the basis of the top peak value and the bottom peak value of the servo error signal which are stored in the storage means 7, and obtains an offset adjustment value which cancels this difference, and stores the offset adjustment value in the storage means 7.
As described above, the conventional optical disk drive performs offset adjustment and gain adjustment for the focusing and tracking servo system during reading or writing of the optical disk 3, thereby controlling the servo system so that the objective lens in the optical pickup 1 is brought into an optimum state with respect to the optical disk 3.
In the conventional technique described above, however, when performing writing on a recordable optical disk such as a CD-R (Compact Disc-Recordable) or a CD-RW (Compact Disc-ReWritable), the gain of the servo loop is temporarily increased due to a response delay from when the laser power switching circuit changes the laser power to when the AGC completes gain control, whereby the servo system becomes unstable, resulting in the fear of off-servo. The influence might be increased especially when the laser power is changed at high speed as in the OPC (Optimum Power Control) operation seeking for the optimum writing power, or when high-speed writing on the optical disk is carried out by the improved technique in the recent development of optical disk drives.
Further, since, in the conventional method, offset capturing is carried out by alternately switching the laser output power between the reading power and the erasing power, data written on the disk might be destroyed.
Furthermore, in the writing operation on the CD-RW disk, because it is necessary to servo-control the optical pickup on the basis of the detection signal of the reflected light from the disk at the erasing power of the laser, an offset adjustment value for writing is required aside from an offset adjustment value for reading during which the optical pickup is servo-controlled with the detection signal of the reflected light from the disk at the reading power. Moreover, in the push-pull method as the tracking error signal generation method, since an optical offset may occur due to deviation of the optical axis of the photodetector as a light-receptive element, an offset adjustment value for writing must be obtained considering the optical offset.
The present invention is made in view of the above-mentioned situation and has for its object to provide an optical disk drive which can quickly perform gain control to maintain the stability of the servo system, even when the gain of the servo system changes significantly when performing writing on the optical disk.